The invention relates to a method and apparatus for providing optimum radar elevation patterns at long and short ranges, and more specifically, to a method and apparatus for transmitting high energy pulses, also referred to as long pulses, in a first optimum beam pattern for detection of targets at long ranges, and transmitting low energy pulses, also referred to as short pulses, in a second optimum beam pattern for detection of targets at short ranges, in surveillance radar using solid-state transmitters.
Solid state transmitters are gradually replacing magnetron and klystron transmitters. Although providing numerous advantages over magnetron and klystron transmitters, solid state transmitters have one drawback in that they generate low peak power. Consequently, sufficient energy for long range detection can be provided only by transmitting long pulses. However, the longer the pulse, the farther away the object must be to ensure that its echo is not distorted by a transmitted pulse unavoidably present in the receiver. For example, if a pulse of 100 .mu.s is transmitted, the target must be at least 8 nautical miles (nmi) away from the transmitter before its entire echo can be received undistorted. Exceedingly strong short range clutter echoes complicate reception such that only echoes beginning several nautical miles beyond the end of the transmission can be received without distortion.
Long pulses for solid-state radars can be coded to provide the bandwidth necessary for the desired range accuracy and resolution. Echoes received are decoded or "compressed" into short pulses, but some undesired energy known as "range sidelobes" extend as much as the transmitter pulsewidth to either side of the desired short pulse, obscuring weaker echoes from neighboring targets. If the uncompressed echo is distorted by strong overlapping interference (transmitted pulse or short range clutter echoes) exceeding the linear dynamic range of the receiver, the range sidelobes grow larger, making it impossible to detect the presence of a small aircraft at the same azimuth as a large aircraft, unless their range separation exceeds the transmitter pulsewidth. This is unacceptable for airport surveillance radar (ASR), where collision avoidance between large commercial airliners and small general aviation aircraft is of prime concern.
Thus, since long pulses cannot be utilized to accurately detect targets at short ranges, it is necessary to transmit separate long and short pulses to provide long range and short range coverage, respectively. Examples of ASR systems utilizing this technique are the RAMP PSR manufactured by the Raytheon Corp. and the AN/TPS-73 manufactured by the Selenia/Unisys Corps.
FIG. 1 is a graph showing exemplary coverage patterns for the RAMP PSR. The coverage patterns show target locations where the circularly polarized radar provides 80% probability of detection of a two square meter target. Coverage patterns 10 and 14 result from transmission and reception on the same beam. Coverage pattern 12 results from reception on a higher elevation beam and cannot be employed past 20 nmi without losing coverage of low altitude aircraft.
The long range pulses, corresponding to beam patterns 10 and 12, are 100 .mu.s in length, and the short range pulses, corresponding to coverage pattern 14 are 1 .mu.s in length. The long range coverage pattern presumes no ground clutter interference. At short range, echoes from terrain or sea create clutter interference which must be suppressed by the use of filters which reject their low Doppler frequencies, sometimes called moving target indicators (MTI). As is well known in the radar art, Doppler filters introduce losses in sensitivity, even when they are able to attenuate clutter echoes well below receiver noise.
A first loss, estimated to be 4 dB in computing coverage pattern 14, is caused by the correlation of receiver noise by the Doppler filter, reducing the effectiveness of integrating the multiple samples received during the time that the antenna beam dwells on a target. The reduction of the effective number of pulses integrated causes an increase in the average echo power required to achieve a given detection probability. This sensitivity loss is exaggerated by signal processing to control the false alarm rate in rain or jamming environments.
A second loss is created when the target has an unfavorable Doppler frequency, even though outside the clutter rejection notch. The gain of the Doppler filter varies as a function of the target range rate. This is known as velocity response. "Blind speeds" are those speeds with response more than 20 dB below average, and "dim speeds" create more modest loss. Operational utility of a radar depends on detecting targets at a large fraction (90-99%) of possible range rates, therefore, velocity response loss shrinks coverage 14.
The peak power of the signal used for both the long and short pulses is the same. However, the short pulse contains approximately 20 dB less energy. The short pulse sensitivity, including 4 dB average Doppler filtering loss, is 24 dB less than the long pulse, shrinking the coverage of the short range beam pattern in both range and altitude by a factor of 4. As a result, as is apparent in FIG. 1, an area 16 exists in the coverage pattern above approximately 10,000 feet for distances within approximately 10 nmi of the radar transmitter where neither the long range nor short range beam patterns provide aircraft detection. This area 16 is referred to herein as a "hole" in the coverage pattern. Any "hole" in the coverage pattern of a radar system is, of course, a serious problem.
As described, the coverage pattern 14 in FIG. 1 and to a two square meter aircraft with a range rate resulting in 0 dB signal-to-noise gain from the Doppler filter. A smaller aircraft or a range rate corresponding to a "dim speed" would create a much larger "hole" in the critical coverage region of an ASR system.
The beam coverage pattern 10 shown in FIG. 1 also exaggerates clutter interference, providing about 22 dB higher gain to hills in the nose of the beam patterns than to aircraft at elevation angles between 15.degree. and 35.degree.. Receiving the long range pulse over a higher elevation pattern, such as beam pattern 12, is helpful in reducing this clutter exaggeration in the 10 to 30 nmi range, but it is substantially ineffective in the short range region. In particular, at approximately the 5 nmi range, a hill has to be only 1,500 feet above the airport (approximately 3.degree. of elevation) to create stronger clutter echoes in the high beam 12 than in the low beam 10.
The only known prior solution to correct for inadequate range coverage is to increase the peak power of the transmitted pulses by about 40%. This permits the long pulse length to be shortened to approximately 70 .mu.s which moves the long range beam coverage closer to the transmitter to about 7.5 nmi, and provides sufficient energy to the short pulse to cover out to about 7.5 nmi for a specified aircraft cross-section at average Doppler. However, increasing the peak power of the transmitted pulses significantly increases the cost of the transmitter. Also, provision of an adequate safety factor of power to cope with "dim speeds" or smaller aircraft cross-sections becomes very costly and causes even more saturation of clutter echoes.